Crystalline polymers represented by polyolefin-based polymers are used widely as films, sheets, structural parts, or the like, because of their excellent moldability, thermal stability, and low specific gravities. However, transparency of the molded articles of the crystalline polymers is not necessarily good.
It is known that this defect arises from the crystallizing property of the crystalline polymer and that transparency of the crystalline polymer is improved by addition of a crystal nucleator, whereby the crystallizing temperature of the crystalline polymer is raised and crystallization thereof is accelerated to form fine crystals.
As the crystal nucleators, there are used, for example, a metal carboxylate such as sodium benzoate, aluminum 4-tert-butylbenzoate, sodium adipate, disodium bicyclo[2.2.1]heptane-2,3-dicarboxylate; a metal salt of phosphate ester such as sodium bis(4-tert-butylphenyl)phosphate and sodium 2,2′-methylene-bis(4,6-di-tert-butylphenyl)phosphate; a polyalcohol derivative such as dibenzylidene sorbitol, bis(methylbenzylidene) sorbitol, and bis(dimethylbenzylidene) sorbitol; and the like.
Among these compounds, especially, a metal salt of cyclic phosphate ester typified by 2,2′-methylene-bis(4,6-ditert-butylphenyl)phosphate exhibits a great effect in improving the properties of crystalline polymer material such as mechanical characteristics and transparency. Patent Documents 1 to 6 disclose that by using a metal aliphatic carboxylate together with the metal salt of cyclic phosphate ester, even better effect is obtained. Furthermore, Patent Documents 3 to 6 disclose that a crystalline polymer material added with a co-pulverized mixture containing a metal salt of cyclic phosphate ester which is a crystal nucleator ingredient and a metal aliphatic carboxylate exhibits an effect specifically in improving transparency.
Furthermore, to prevent the coagulation of crystal nucleator, Patent Documents 7 and 8 disclose a method in which the crystal nucleator, a metal salt of cyclic phosphate ester, is supported on the surface of inorganic material. Also, Patent Document 9 discloses a method of preventing the coagulation of inorganic fine particles, in which inorganic fine particles and inorganic pigments are mixed and pulverized together with a wet pulverizer to give inorganic fine particles supported on the surface of inorganic pigments.
As described above, various methods have been proposed to improve the effect of crystal nucleator in imparting transparency and the dispersibility of fine particles. However, as regards the metal salt of cyclic phosphate ester used as a crystal nucleator, the methods described above do not provide sufficient results and there still remain the following problems.
With the production methods described in Patent Documents 3 to 6, in which the crystal nucleator is co-pulverized with a metal aliphatic carboxylate, there is a problem that the pulverized material sticks together in the pulverizing vessel to lower productivity. There is also a problem that many fisheyes are formed in a crystalline polymer material containing the crystal nucleator to damage the appearance of a polymer. Regarding the production methods described in Patent Documents 7 to 9, there are problems that a drying step of pulverized material is necessary because the methods involve a wet pulverizing step, and moreover, it is difficult to obtain the effect of pulverization because pulverized material reaggregates upon drying.
Patent Document 1: Japanese Patent No. 3058487
Patent Document 2: Japanese Patent Laid-Open Publication No. 2002-338820
Patent Document 3: Japanese Patent Laid-Open Publication No. 2003-313444
Patent Document 4: Japanese Patent Laid-Open Publication No. 2003-335968
Patent Document 5: Japanese Patent Laid-Open Publication No. 2004-083852
Patent Document 6: Japanese Patent Laid-Open Publication No. 2005-054036
Patent Document 7: Japanese Patent Laid-Open Publication No. 2001-026666
Patent Document 8: Japanese Patent Laid-Open Publication No. 2003-012868
Patent Document 9: Japanese Patent No. 3606400